When I finish reading a book, I attach a sticker on the inside cover with a little ad for postcrossing (as well as for swap-bot and bookcrossing). I then leave these books in Little Free Libraries throughout my city.
I’m just trying to spread information about this hobby, via one book / person at a time 
Now, this is a great idea, thank you! I have a bunch of books that I have to leave in bookcrossing zones - I’m going to put a postcard with a little ad about Postcrossing in each one of them 
What a great idea…but it is allowed to do so?
I hope so. It’s only a note singing the joys of Postcrossing. The finder can choose to ignore, leave the card where they found it or throw it away.
I think it’s not a problem as long as you don’t use the Postcrossing logo. I’m thinking to use some old postcards and write something along the lines of “Do you know Postcrossing? It’s a project that allows you to exchange postcards with people from all over the world” and then add the website link.